Water Balance
Toolbox (v.2) for ArcGIS

James
DyerProfessor of Geography

A water balance (or budget) explores the interactive relationship
between energy and moisture at a place, by modeling moisture
demand (potential evapotranspiration) and supply (precipitation
and soil moisture storage). The ArcGIS tool available here
computes a complete monthly water balance for each pixel within a
digital elevation model, using readily-available data layers.

The water balance model was first presented in the following
article (click to download
a copy):
Dyer, J.M. 2009. Assessing topographic patterns in moisture use
and stress using a water balance approach, Landscape Ecology
24: 391-403.
The web
page for version 1 of the model is still available; however,
Version 2, described below, offers numerous improvements:

more efficient code to speed up
processing

the ability to run routines for an
entire year instead of a month-by-month basis

extensive annotation of the models in
case users wish to modify the routines

inclusion of a tool for computing
Potential Evapotranspiration (PET) in semi-arid or arid
climates (relative humidity <50%) using an adjusted Turc
equation

a PET Adjust toolset to account for
temperature changes throughout the day. (With a
monthly time-step, there is no diurnal variation in
temperature, and so maximum PET occurs on southern
exposures, since that is where maximum insolation occurs;
PET is then symmetrical about the N-S axis.) This
toolset employs “adjustment coefficients” that either
increase or decrease PET based on topographic position.

Potential evapotranspiration modeled
at Asheville NC, using the Water Balance Toolbox.
Circular charts show how average PET varies with slope
and aspect throughout the year. Aspect is
represented along the circumference, and slope
steepness increases from the center to the edges of
the circles.

Available water capacity (top 1
meter of soil) and monthly deficit for the Vinton
Furnace Experimental Forest (Vinton County, OH),
modeled with the Water Balance Toolbox.

Model Details: The water
balance tool computes potential evapotranspiration using the Turc
method. The toolbox then computes soil moisture storage, actual
evapotranspiration, soil moisture deficit, and soil moisture
surplus for every grid cell within a DEM, using the
Thornthwaite-Mather approach. Data needs for performing a water
balance using GIS are few: a digital elevation model (DEM), soil
available water capacity (AWC), and monthly temperature,
precipitation, and solar radiation. If you use this model in your
research, please acknowledge the source and cite the original
publication (above). I welcome any suggestions, corrections,
or comments on the model: dyer@ohio.edu

Water
Balance Toolbox for ArcGIS(The "Start_Template.mxd" project contained in
the zipped file was created using ArcGIS v. 10.1; if you are
using an earlier version of ArcGIS, you'll want to use the Start
Template for v.9.3 or 10.0. Save them
to the C:\WB\ folder. You may need to manually add the
Water Balance Toolbox to the project. Contact me if you
need additional information.)

ArcGIS provides a “Solar
Radiation” toolset that can compute monthly values of total
(global) radiation for each pixel in a DEM, based on slope,
aspect, topographic shading, latitude, and time of
year. The user must specify two atmospheric
parameters: the diffuse proportion of global radiation, and
transmittivity (the proportion of solar radiation outside
the atmosphere that reaches the surface). The
toolset’s default values for these parameters are 0.3 and
0.5, respectively. These values can assume a wide
range of values, however. In the eastern US, I have
observed 21 different combinations, with monthly values of
diffuse proportion between 0.2 – 0.7, and transmittivity
values between 0.3 – 0.7. Misspecifying these values
can have profound effects on radiation estimates.
Since additional factors, such as surface reflectance or
altitude, may affect radiation at a site, I adopt the
approach of adjusting the two values to best approximate a
“known” radiation value.

Moisture Demand (PET) is
governed by temperature and radiation. But by using a
monthly time-step, diurnal variations in temperature are not
considered. The result is that maximum PET occurs on
southern exposures, since that is where maximum insolation
occurs (and PET is symmetrical about the N-S axis). To
overcome this, the Water Balance toolset employs “adjustment
coefficients” that either increase or decrease PET based on
topographic position. The user has the option of using
PET Adjustment Coefficients derived for four sites in the
eastern U.S., or generating his/her own coefficeints using
these guidelines. Alternatively, this adjustment can
be omitted, allowing maximum PET to occur on southern
aspects.

The National Renewable Energy
Laboratory provides solar
radiation estimates for the U.S. at 10-km resolution, and
"Typical
Meteorological Year" radiation data. Hourly estimates of global
horizontal radiation can be summed to provide a monthly total,
which will be directly comparable to monthly estimates derived
from the Solar Radiation Toolset. See the
information under "Guidelines for parameterizing the diffuse
proportion and transmittivity values for ArcGIS Solar Radiation
tool" above.

Available Water Capacity

In the U.S., soil water-holding
capacity is available from the Natural Resources
Conservation Service's Web
Soil Survey. Detailed instructions on downloading and
preparing soil available water capacity (AWC) grids are provided
in Appendix A of the User Manual (above).

Digital Elevation
Models

DEMs are available for many
locations at ⅓-arc second (~10 m) resolution (or finer) from the
U.S. Geological Survey’s National Map
Viewer. The DEMs that you download will have
elevation in meters, with a Geographic Coordinate System (NAD83)
projection (units = degrees). Thus, the x, y, and z values
will be in different units. You will need to reproject the
grid to a "meter-based" projection (such as Plate Carree
(world), or UTM). Part II of "Guidelines
for parameterizing the diffuse proportion and transmittivity
values for ArcGIS Solar Radiation tool" provides
sample instructions
for downloading and projecting DEMs.

Climate

Climate data for individual stations (including
climate normals) are available from the National
Climatic Data Center. Gridded monthly temperature and
precipitation data are available for the U.S. at 30-arc second
(~800 m) resolution from the PRISM Group. After downloading and unzipping the ".asc"
grid, use ArcGIS's ArcToolbox – Conversion Tools – To Raster
– ASCII to Raster. The new grid will be in
Geographic Coordinate System - Spheroid-Based - GRS 80
(ArcToolbox – Data Management Tools – Projections and
Transformations – Define Projection).